PCT publication no. WO 03/080905A discloses an electroblowing apparatus and method for producing a nanofiber web. The method comprises feeding a polymer solution to a spinning nozzle to which a high voltage is applied while compressed gas is used to envelop the polymer solution in a forwarding gas stream as it exits the spinning nozzle, and collecting the resulting nanofiber web on a grounded suction collector.
There are several disadvantages to the apparatus disclosed in PCT publication no. WO 03/080905A, particularly if the process is carried out on a commercial scale. For one, the spinning nozzle, and the spinneret and spin pack of which the nozzle is a component and all of the associated upstream solution equipment must be maintained at high voltage during the spinning process. Because the polymer solution is conductive, all of the equipment in contact with the polymer solution is brought to high voltage, and if the motor and gear box driving the polymer solution pump are not electrically isolated from the pump, a short circuit will be created which will reduce the voltage potential of the pack to a level insufficient to create the electric fields required to impart charge on the polymer solution.
Another disadvantage of the prior art electroblowing apparatus is that the process solution and/or solvent supply must be physically interrupted in order to isolate it from the high voltage of the process. Otherwise, the solution and/or solvent supply systems would ground out the pack and eliminate the high electric fields required for imparting charge on the polymer solution.
Additionally, all of the equipment in contact with the electrified polymer solution must be electrically insulated for proper and safe operation. This insulation requirement is extremely difficult to fulfill as this includes large equipment such as spin packs, transfer lines, metering pumps, solution storage tanks, pumps, as well as control equipment and instrumentation such as pressure and temperature gauges. A further complication is that it is cumbersome to design instrumentation and process variable communication systems that can operate at high voltages relative to ground. Furthermore, all exposed sharp angles or corners that are held at high voltage must be rounded, otherwise they will create intense electric fields at those points that may discharge. Potential sources of sharp angles/corners include bolts, angle irons, etc.
Moreover, the high voltage introduces a hazard to those persons providing routine maintenance to electrified equipment in support of an on-going manufacturing process. The polymer solutions and solvents being processed are often flammable, creating a further potential danger exacerbated by the presence of the high voltage.
Another disadvantage of the prior art electroblowing apparatus is the necessity of using a quite high voltage. In order to impart electrical charge on the polymer, an electrical field of sufficient strength is needed. Due to the distances involved between the spinning nozzle and the collector, high voltage is used to maintain the electric field. An object of this invention is to lower the voltage used.
Still another disadvantage of the prior art electroblowing apparatus is the coupling of the spinning nozzle to collector distance to the voltage used. During operation of the prior art process, it may be desirable to change the distance of the spinning nozzle to the collector (or the die to collector distance; the “DCD”). However, by changing that distance the electric field generated between the spinning nozzle and the collector changes. This requires changing the voltage in order to maintain the same electric field. Thus, another object of this invention is to decouple the spinning nozzle to collector distance from the electric field strength.
In co-pending U.S. patent application Ser. No.11/023,067, filed Dec. 27, 2004, which is incorporated herein by reference in its entirety, an improvement to the apparatus and process of PCT publication no. WO 03/080905A is disclosed, which discloses an alternative charging method for an electroblowing process and apparatus, which also permits decoupling of the DCD from the electric field strength.
U.S. Pat. No.4,215,682 discloses an apparatus for imparting a persistent electrical charge to melt-blown fibers to form electret fibers, wherein the charging apparatus comprises at least one electrical source in the form of a wire, which is charged to a voltage high enough to form a corona around the source. The melt-blown fibers pass the electrical source and through the corona to form electret fibers with a persistent electrical charge.